Flight Control for Automatic Aerial Refueling via PNG and LOS Angle Control

Abstract

Aerial refueling is in -flight fuel transfer from a tanker aircraft to a receiver a ircraft. It is a problem of intercept or rendezvous, being considered as one of the most difficult tasks for pilots. The problem will remind one of missile guidance, where the proportional navigation guidance (PNG) is commonly used, and approach guidance f or instrument landing system (ILS) landing, where line -of -sight (LOS) angle is precisely controlled. The latter is similar to pure -pursuit guidance for missiles. This observation takes the authors to the idea of flight control for automatic refueling based on the PNG and the LOS angle control. Unlike missile guidance and approach guidance, the airspeed of the receiver aircraft must be decreased to that of the tanker. In order to control the airspeed, forward acceleration command is first generated using the relative speed and range between the receiver and the tanker, and then integrating the acceleration command creates the forward speed command. The flight control systems (FCS) are designed for longitudinal motion and lateral -directional motion separately. In the PNG -based method, the longitudinal FCS controls upward acceleration and airspeed using the elevator and engine thrust, and the lateral -directional one controls sideward acceleration and sideslip angle using aileron and rudder. In the LOS -angle -ba sed method, two variables which are defined to be integrals of flight path angle and flight directional angle are introduced to avoid high sensitivity of the LOS angle to variation of relative position of the aircraft. The FCSs control those variables alo ng with the airspeed and sideslip angle. Since the control problem is inherently a multi -input multi -output one, the authors have employed a state -space design method, that is, the model following servo controller, which is a kind of integral -type state -feedback optimal servomechanism. To illustrate the effectiveness and compare control performance of both methods, computer simulation has been conducted using a linear model of a jet trainer for the receiver aircraft. Although the aircraft model is linear , nonlinearity in kinematic and geometric relations is included in thesimulation. Simulation results show that both methods have good control performance under the circumstances without air turbulences. However, there can actually exist air turbulences; particularly, neglecting the effect of vortex wake by the tanker is impractical. Therefore, computer simulation has been conducted also in the presence of the turbulences, which are given by a Dryden wind turbulence model. It turns out that it is difficul t to settle the aircraft at a target position precisely and it is likely that the aircraft deviates vertically and horizontally several meters from the desired position. Those deviations sometimes cause the guidance methods to fail due to the increased se nsitivity of the LOS angle/rate; especially, the PNG -based method is more sensitive to the turbulences. To solve the problem, the FCSs are switched to those controlling relative vertical and horizontal distances, when the forward distance becomes less tha n 50m. Although the distance control makes the FCSs robust, the receiver still deviates around the target position; however, simulation results show that the control provides a chance that the probe is connected to the drogue, even if it happens by chance.